1. Field of the Invention
This invention relates to a method and apparatus for guiding optical signals by controlling the length of waveguides used to guide the optical signals.
2. Background of Related Art
A waveguide grating router developed by C. Dragone as disclosed in U.S. Pat. No. 5,136,671, incorporated herein by reference, is a device with N.sub.1, inputs and N.sub.2 outputs that consists of M grated waveguides (i.e. "arms") of varying lengths connecting two star couplers. Following the work of Dragone, and limiting the discussion to an N.times.N router with input channels evenly spaced in frequency and having free-spectral ranges equal to N times the channel spacing, the amplitude transmission between port p at one end of the router and port q at the other can be written as ##EQU1## where f is the optical frequency, P.sub.m is the optical power in the mth arm normalized to the total power, n is the effective index of refraction of the waveguides, c is the speed of light in vacuum and l.sub.m is the length of the mth arm of a total, M, arms. In a simple router design, l.sub.m =m.DELTA.l+C, where .DELTA.l and C are constants. The offset value C does not affect router response and is left out in all following equations. The value .vertline.t.sub.p,q .vertline..sup.2 in such a router has equal-height transmission peaks, i.e., passbands, that occur at f=k-(p-q)/N!c/(n.DELTA.l), where k is an integer.
The router itself acts as a filter for each input-output (p-q) combination. When one port or ports (e.g., q) are terminated with an array of N amplifiers and mirrors and a mirror and possibly an amplifier on the other side (i.e. p side), a multi-frequency laser (MFL) oscillating at N precisely spaced frequencies is obtained.
Within each channel however, a signal having a peak power level will repeatedly occur at different frequencies determined by the cavity's free-spectral range. Therefore, in some of the channels a net power gain may be nearly the same for two or more frequencies, assuming any optical amplifiers connected to the MFL router all have the same characteristics. This repetition of a peak signal within each channel results in multimode lasing of an optical laser which produces instabilities in the laser's output.
A MFL router having only one dominant passband (i.e. frequency range) or peak signal is therefore highly desirable. In addition, it is desirable to control the frequency or passband at which the peak signal occurs within each channel.
It should be noted that where it is used herein, the word "frequency" may denote a signal having a single frequency or a multitude of frequencies, i.e. a passband.